The present invention relates to an organic thin film element. 2. Description of the Related Art
In recent years, attention has been increasingly paid to molecular electronics using various physical properties of organic molecules to realize devices having novel functions not obtained by conventional semiconductor devices. For example, studies for applying organic molecules to a non-linear optical element, a switching element, and an electric-field light-emitting element have been actively made. From the viewpoint of the application of organic molecules to these elements, especially a charge-transfer phenomenon occurring between organic molecules has attracted attention.
Organic materials consist of donor molecules each having a low ionization potential and a tendency to give an electron to another molecule to become a positive ion and acceptor molecules each having a high electron affinity and a tendency to receive an electron from another molecule. It is well known that a compound called a charge-transfer complex is formed between these two types of molecules. For example, a compound of perylene and tetracyanoquinodimethane (TCNQ) consists of neutral molecules. On the other hand, a compound of tetramethylphenylenediamine (TMPD) and TCNQ is an ionic compound in which molecules of the respective substances become positive and negative ions. It is also known that a neutral-ionic transition phenomenon according to a change in temperature or pressure is observed in a compound of tetrathiafulvalene (TTF) and chloranil (CA) (J. B. Torrance et al.: Phys. Rev. Lett., 46, 253 (1981).
To apply the charge-transfer phenomenon in organic molecules as the operational principle of an electrical or optical element, it is important to cause charge-transfer using an electric field or light with high efficiency and controllability. Data of interest, relating to electrical characteristics of the charge-transfer complex, has recently been reported (Yoshiki Tokura et al.: Manuscripts for Meeting of the Physical Society, 3a-S4-1, 3a-S4-2, 3a-S4-3, etc. (Autumn 1988); Y. Tokura et al.: Physica 143B, 527 (1986)) Namely, it is reported that in a mixed stacked complex crystal in which donor molecules and acceptor molecules are stacked with their molecular planes facing each other, the anisotropy of relative dielectric constant is high, the relative dielectric constant in the direction of stacking is very high, i.e., 100 to 1,000, and non-linear electric conductivity or switching characteristics are observed under an electric field on the order of 10.sup.3 to 10.sup.4 V/cm. The reason for this is assumed that an ionic domain thermally or electrically formed in a neutral crystal or a neutral domain formed in an ionic crystal is dynamically moved by an electric field.
This phenomenon, though relating to neutral-ionic transition, occurs in a very local area, and no macroscopic change appears in the whole crystal. No macroscopic neutral-ionic transition caused by an electric field or light has yet been realized.
To cause a macroscopic neutral-ionic transition in a charge-transfer complex using an electric field, it is very important that the direction of the electric field in an element coincides with the direction of a stacking axis of donor and acceptor molecules. To realize devices effectively using the characteristics of organic molecules as described above, control of a molecular structure on the order of a single molecule in a film, a mutual arrangement between neighboring molecules, and a molecular orientation is very important as well as dimensions such as a film thickness and structural uniformity.
Recently, as a method of manufacturing a very thin film having controlled molecular orientation and structure, a Langmuir-Blodgett (LB) method has become a big concern. In this method, monomolecular films formed on a water surface are accumulated on a substrate one by one to form superlattice films of the same type or different types. Actually, however, a packing state or uniformity of molecules in a film developed on the water surface is poor, and a monomolecular film structure is disturbed when it is accumulated on the substrate. Therefore, this method has not yet reached the level capable of forming a superlattice thin film having a controlled molecular orientation in the entire film or between the accumulated layers. To improve the film formation techniques according to the LB method, it is necessary to design molecules suitable for the LB method and to establish a synthesizing technique for such molecules.
On the other hand, as a technique requiring no specific molecular design and capable of being easily applied to most of organic molecules, a vacuum deposition method has been actively studied. In the vacuum deposition method, however, a molecule evaporation source is temporarily gasified and aggregated again. Therefore, it is predicted that a film structure or a molecular orientation variously changes depending on a balance between the supply rate of gasified molecules, the rate of surface diffusion or crystallization of molecules deposited on a substrate surface and an interaction between adsorbed molecules and the substrate surface.
In the conventional studies about an organic deposited film, a thin film growth process and a molecular orientation of mainly long-chain hydrocarbon-based linear molecules or tabular molecules such as phthalocyanine on various types of substrates have been examined. A single-crystal alkali halide or a single-crystal metal, quartz, and an Si single crystal are mainly used as substrates to execute evaluation using an electron microscope or electron-beam diffraction, optical evaluation, and electrical evaluation, respectively. As the deposition conditions, influences of a substrate temperature and a deposition rate have been checked in many cases. Vincett et al. in U.S.A. has reported that a uniform continuous film can be formed by setting a substrate temperature (on the absolute temperature scale) to be 1/3 the boiling point of a deposition material regardless of the types of the deposition material and the substrate. However, it is very difficult to control the orientation of an organic thin film deposited on a given substrate simply by setting proper deposition conditions.
The following reports are known as researches on influences of a substrate on the molecular orientation of an organic thin film formed thereon. (1) Karl et al. in West Germany have reported that in a perylenetetracarboxy dianhydride deposition film of several molecular layers formed on a pure Si single-crystal surface, molecular planes are oriented parallel to the substrate surface. (2) Hara has reported, in the research on a phthalocyanine deposited film using a molecular-beam deposition method, that even under the conditions in that only a discontinuous film can be formed by normal high-vacuum deposition, a uniform continuous film in which molecular surfaces are oriented parallel to a substrate surface can be formed at a very low deposition rate of about 0.1 nm/min under vary high vacuum. In this research, to avoid lattice mismatching between an inorganic substrate and deposited organic crystals, MoS.sub.2 which is a layered compound is used as a substrate on the basis of the idea of Van der Waals epitaxy. (3) Harada et al. have reported that molecular planes are oriented parallel to a substrate surface in a pentacene deposited film of several molecular layers formed on a graphite substrate.
Although the various types of studies have been made on thin film formation according to the vacuum deposition method as described above, control factors of the film structure and the molecular orientation have not yet been totally understood.
The present inventors have made extensive studies with an emphasis on an interaction between a substrate and deposited molecules. As a result, the present inventors have found that, when a film of aromatic group-based molecules is to be deposited, a uniform continuous film can be easily formed by using a highorientation graphite substrate having high similarity to a deposited film in terms of a chemical structure and an electronic structure and setting proper deposition conditions. In addition, it is assumed that a factor determining the orientation of an aromatic group-based molecules deposited on a high-orientation graphite substrate is a Van der Waals interaction occurring between .eta. electrons on the surface of the graphite substrate and those of the aromatic group-based molecules. That is, it is assumed that the molecules are stabilized most in terms of energy by this interaction while their main molecular planes are oriented parallel to the substrate. However, it is difficult to form an element as an electronic or optical device using a graphite substrate.
As described above, to cause a neutral-ionic transition according to an electric field in a charge-transfer complex of donor and acceptor molecules, the direction of stacking axis of the complex must coincide with the application direction of the electric field. However, it is very difficult to control the orientation of an organic thin film on a given substrate simply by setting proper thin film formation conditions. If a graphite substrate is used and the formation conditions of an organic thin film are properly set, the orientation of the organic thin film can be easily controlled. However, it is difficult to form an element as an electronic or optical device using a graphite substrate.